Where re is the input impedance of the circuit. Voltage Ii Vi produces a change in emitter current AlE given by Voltage is developed across the load resistor RL. The input voltage is applied between the emitter and the base, while the output This circuit is known as the commonīase circuit due to the fact that the base terminal is common to both input The basic amplifying action may be understoodīy considering the circuit of Fig 6.1. The transistor is widely used as an amplifying device, and can provide voltage The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of “1” (unity) or less, in other words the common base configuration “attenuates” the input signal. The base terminal is grounded or can be connected to some fixed reference voltage point. The input signal is applied between the transistors base and the emitter terminals, while the corresponding output signal is taken from between the base and the collector terminals as shown. Now let's say we remove the voltage between base and collector hence the circuit looks as follows.Ĭommon Base Configuration Īs its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal. See the image below for active biasing of PNP transistor. It's in the active region that transistor acts as a amplifier and so on. When emitter-base is forward biased and collector based is reversed bias, the transistor is said to be in active region. One can see that its emitter is held more positive than base and base more positive than collector. NPN transistor works the same way as PNP, but with voltage and currents reversed.īelow is the diagram of PNP transistor. To see how transistor operates we will look at how an PNP transistor works. In a similar way by sandwiching a lightly doped P region between highly and moderately doped N region we get a NPN transistor as shown below. The total width of the transistor will be 150 times that of the width of the base. In a real transistor, the width of base is very thin. The doping of base is just one tenth of that of collector. The middle region is called the Base, and it's this region that serves as a gate, regulating flow of charge from Emitter to collector. The minus'-' sign indicates it's doped very very less compared to the emitter and collector. The middle region is denoted by n- because it's doped with N-type impurities. This portion collects the majority charge carriers that is been emitted by Emitter and that manage to cross the collector. At the extreme right is moderately doped P type material which is called as the Collector. This highly doped portion is called Emitter, that is the piece of semiconductor that supplies majority carriers for the transistor to function. The left part is indicated by P+ which means its highly doped P-type material. As you can see an N-type silicon (green layer) is sandwiched between two P type materials (red layer). The above diagram shows the schematic construction of a PNP transistor. Or placing n-type material between two p-type material which forms the pnp-transistor or by placing a p-type material between two n-type semiconductor which forms npn-transistor. Transistor Construction Ī transistor is constructed by placing a oppositely doped semiconductor material between two similarly doped semiconductors. 10 Transistor Casing and Terminal Identification.
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